It can undo you all over again — the other animal going room to room, sitting by the door at the hour the other one used to come home, sniffing the empty bed, eating less, going quiet. You are grieving, and now there is a second grief in the house, one that can’t be explained to the creature carrying it. You can’t sit them down and tell them what happened. You just have to watch them not understand.
Animals who lived alongside each other do seem to register the absence — the broken routine, the missing smell and sound and weight of another body in the room. Whether what they feel is grief in the way you mean the word, no one can say for certain, and anyone who tells you they know is guessing. But you don’t need the science settled to see that something in your animal has changed, and that watching it hurts — partly because it reflects your own searching straight back to you. They are doing on four legs what you are doing in your chest: waiting for someone who isn’t coming.
There is a strange, real comfort buried in it, even so. You are not the only one in the house who loved them. The grief the world keeps trying to tell you is too much — here is another living thing that plainly felt the loss too, that isn’t performing it or measuring it or wondering when you’ll be over it, just living inside it alongside you. Sometimes the surviving animal is the only one who seems to understand the actual size of what’s gone.
They will, in time, settle — animals tend to find their way back to eating and sleeping and the ordinary rhythms, often sooner than you do. That isn’t them forgetting, any more than your own quieter days will be forgetting. And in the meantime, the two of you are keeping each other company inside the same absence. You don’t have to carry it for both of you. You’re carrying it together, in the only way the two of you can.